I frequently encounter this problem with clients and arguably this is a large reason why I am in business all together because this can be a bit tricky to get just right.
The problem with ‘green water’ is not actually a problem with the color, but with the amount of suspended single-cell algae. The issue is with filtration and the abundance of algae-food. To resolve this issue, try stocking Filter Grass if not already present. Also, it may be necessary to reduce the number of fish as this will reduce the amount of fish waste. If the green water becomes a persistent problem consider having an UltraViolet filter installed. When the single algae cells are exposed to Ultra-Violet rays the cells are destroyed and you are back to transparent water once again.